Multisensory Effect Research Articles

Assessing the Effects of Exercise, Cognitive Demand, and Rest on Audiovisual Multisensory Processing in OlderAdults: A Pilot Study.

A single bout of aerobic exercise is related to positive changes in higher-order cognitive function among older adults; however, the impact of aerobic exercise on multisensory processing remains unclear. Here we assessed the effects of a single bout of aerobic exercise on commonly utilized tasks that measure audiovisual multisensory processing: response time (RT), simultaneity judgements (SJ), and temporal-order judgements (TOJ), in a pilot study. To our knowledge this is the first effort to investigate the effects of three well-controlled intervention conditions on multisensory processing: resting, completing a cognitively demanding task, and performing aerobic exercise for 20 minutes. Our results indicate that the window of time within which stimuli from different modalities are integrated and perceived as simultaneous (temporal binding window; TBW) is malleable and changes after each intervention condition for both the SJ and TOJ tasks. Specifically, the TBW consistently became narrower post exercise while consistently increasing in width post rest, suggesting that aerobic exercise may improve temporal perception precision via broad neural change rather than targeting the specific networks that subserve either the SJ or TOJ tasks individually. The results from the RT task further support our findings of malleability of the multisensory processing system, as changes in performance, as assessed through cumulative probability models, were observed after each intervention condition. An increase in integration (i.e., greater magnitude of multisensory effect) however, was only found after a single bout of aerobic exercise. Overall, our results indicate that exercise uniquely affects the central nervous system and may broadly affect multisensory processing.

Facilitation and interference effects of the multisensory context on learning: a systematic review and meta-analysis.

Although ever-growing literature has documented that a multisensory environment influences individuals' learning, there is less agreement as to whether it facilitates or hinders and why. In this work, we addressed this issue with a systematic review and meta-analysis and examined the multisensory facilitation and interference of learning. The present study reviewed the literature on audiovisual learning and quantitatively synthesized a total of 29 studies with 198 comparisons to (1) assess the effect of multisensory context by examining the role of congruency, (2) examine study-level factors-modality, age, and familiarity-that may moderate the effects observed across studies, and (3) discuss possible explanations and implications for theories of attention in learning. The results indicated that the multisensory effect was constrained by the congruency between multimodal information. Compared to a unisensory presentation, a multisensory context enhanced the task performance in the congruent conditions whereas it hindered the performance in the incongruent conditions. Furthermore, the effect of congruency was modulated by the modality of the target stimulus and the age of participants, and the multisensory effect was limited for familiar multimodal associations under incongruent conditions. Importantly, interference was found from the auditory distractors to the visual targets but not from the visual distractors to the auditory targets, in child participants but not in adults, and for the familiar multisensory associations but not for the unfamiliar ones. These results point to a critical role of selective attention in multisensory learning and imply roles of attentional focusing and filtering in processing congruent and incongruent multimodal information.

Audiovisual integration in the Mauthner cell enhances escape probability and reduces response latency

Fast and accurate threat detection is critical for animal survival. Reducing perceptual ambiguity by integrating multiple sources of sensory information can enhance perception and reduce response latency. However, studies addressing the link between behavioral correlates of multisensory integration and its underlying neural basis are rare. Fish that detect an urgent threat escape with an explosive behavior known as C-start. The C-start is driven by an identified neural circuit centered on the Mauthner cell, an identified neuron capable of triggering escapes in response to visual and auditory stimuli. Here we demonstrate that goldfish can integrate visual looms and brief auditory stimuli to increase C-start probability. This multisensory enhancement is inversely correlated to the salience of the stimuli, with weaker auditory cues producing a proportionally stronger multisensory effect. We also show that multisensory stimuli reduced C-start response latency, with most escapes locked to the presentation of the auditory cue. We make a direct link between behavioral data and its underlying neural mechanism by reproducing the behavioral data with an integrate-and-fire computational model of the Mauthner cell. This model of the Mauthner cell circuit suggests that excitatory inputs integrated at the soma are key elements in multisensory decision making during fast C-start escapes. This provides a simple but powerful mechanism to enhance threat detection and survival.

The timecourse of multisensory speech processing in unilaterally stimulated cochlear implant users revealed by ERPs

A cochlear implant (CI) is an auditory prosthesis which can partially restore the auditory function in patients with severe to profound hearing loss. However, this bionic device provides only limited auditory information, and CI patients may compensate for this limitation by means of a stronger interaction between the auditory and visual system. To better understand the electrophysiological correlates of audiovisual speech perception, the present study used electroencephalography (EEG) and a redundant target paradigm. Postlingually deafened CI users and normal-hearing (NH) listeners were compared in auditory, visual and audiovisual speech conditions. The behavioural results revealed multisensory integration for both groups, as indicated by shortened response times for the audiovisual as compared to the two unisensory conditions. The analysis of the N1 and P2 event-related potentials (ERPs), including topographic and source analyses, confirmed a multisensory effect for both groups and showed a cortical auditory response which was modulated by the simultaneous processing of the visual stimulus. Nevertheless, the CI users in particular revealed a distinct pattern of N1 topography, pointing to a strong visual impact on auditory speech processing. Apart from these condition effects, the results revealed ERP differences between CI users and NH listeners, not only in N1/P2 ERP topographies, but also in the cortical source configuration. When compared to the NH listeners, the CI users showed an additional activation in the visual cortex at N1 latency, which was positively correlated with CI experience, and a delayed auditory-cortex activation with a reversed, rightward functional lateralisation. In sum, our behavioural and ERP findings demonstrate a clear audiovisual benefit for both groups, and a CI-specific alteration in cortical activation at N1 latency when auditory and visual input is combined. These cortical alterations may reflect a compensatory strategy to overcome the limited CI input, which allows the CI users to improve the lip-reading skills and to approximate the behavioural performance of NH listeners in audiovisual speech conditions. Our results are clinically relevant, as they highlight the importance of assessing the CI outcome not only in auditory-only, but also in audiovisual speech conditions.

Does response facilitation to visuo-tactile stimuli around a remote-controlled hand avatar reflect peripersonal space or attentional bias?

People react faster to visuo-tactile stimuli presented near the body (i.e., in peripersonal space) than to tactile stimuli presented alone. This multi-sensory facilitation effect has been used as a measurement of peripersonal space. Previous research has reported that peripersonal space representations can be modulated by actively using hand-held tools or disconnected hand avatars. However, previous research has ignored the possibility that the attentional effect of active tool use could affect multi-sensory facilitation. In the present study, we delivered tactile stimuli to participants' left or right hand concurrently with visual stimuli presented near a virtual hand avatar operated by the movements of participants' left or right hand, which was shown far in a virtual environment and disconnected from the body. Participants reacted to tactile stimuli while ignoring the visual stimuli. The results indicated a multi-sensory facilitation effect when tactile stimuli were delivered to the hand used to operate the hand avatar. In contrast, the facilitation was not observed when the tactile stimuli were delivered to the hand that is not operating the hand avatar. These results suggest that the strength of the multi-sensory facilitation effect differed across conditions, even though the visual attention captured around the hand avatar was controlled across conditions. We concluded that the modulation of peripersonal space resulting from using tools or avatars is nearly independent of visual attention captured around tools or avatars.

Multisensory Information Facilitates the Categorization of Untrained Stimuli.

Although it has been demonstrated that multisensory information can facilitate object recognition and object memory, it remains unclear whether such facilitation effect exists in category learning. To address this issue, comparable car images and sounds were first selected by a discrimination task in Experiment 1. Then, those selected images and sounds were utilized in a prototype category learning task in Experiments 2 and 3, in which participants were trained with auditory, visual, and audiovisual stimuli, and were tested with trained or untrained stimuli within the same categories presented alone or accompanied with a congruent or incongruent stimulus in the other modality. In Experiment 2, when low-distortion stimuli (more similar to the prototypes) were trained, there was higher accuracy for audiovisual trials than visual trials, but no significant difference between audiovisual and auditory trials. During testing, accuracy was significantly higher for congruent trials than unisensory or incongruent trials, and the congruency effect was larger for untrained high-distortion stimuli than trained low-distortion stimuli. In Experiment 3, when high-distortion stimuli (less similar to the prototypes) were trained, there was higher accuracy for audiovisual trials than visual or auditory trials, and the congruency effect was larger for trained high-distortion stimuli than untrained low-distortion stimuli during testing. These findings demonstrated that higher degree of stimuli distortion resulted in more robust multisensory effect, and the categorization of not only trained but also untrained stimuli in one modality could be influenced by an accompanying stimulus in the other modality.

Neural Basis of Anticipatory Multisensory Integration.

The brain is able to gather different sensory information to enhance salient event perception, thus yielding a unified perceptual experience of multisensory events. Multisensory integration has been widely studied, and the literature supports the hypothesis that it can occur across various stages of stimulus processing, including both bottom-up and top-down control. However, evidence on anticipatory multisensory integration occurring in the fore period preceding the presentation of the expected stimulus in passive tasks, is missing. By means of event-related potentials (ERPs), it has been recently proposed that visual and auditory unimodal stimulations are preceded by sensory-specific readiness activities. Accordingly, in the present study, we tested the occurrence of multisensory integration in the endogenous anticipatory phase of sensory processing, combining visual and auditory stimuli during unimodal and multimodal passive ERP paradigms. Results showed that the modality-specific pre-stimulus ERP components (i.e., the auditory positivity -aP- and the visual negativity -vN-) started earlier and were larger in the multimodal stimulation compared with the sum of the ERPs elicited by the unimodal stimulations. The same amplitude effect was also present for the early auditory N1 and visual P1 components. This anticipatory multisensory effect seems to influence stimulus processing, boosting the magnitude of early stimulus processing. This paves the way for new perspectives on the neural basis of multisensory integration.

Visual-Tactile Spatial Multisensory Interaction in Adults With Autism and Schizophrenia.

Background: Individuals with autism spectrum disorder (ASD) and schizophrenia (SZ) exhibit multisensory processing difficulties and social impairments, with growing evidence that the former contributes to the latter. However, this work has largely reported on separate cohorts, introducing method variance as a barrier to drawing broad conclusions across studies. Further, very few studies have addressed touch, resulting in sparse knowledge about how these two clinical groups may integrate somatic information with other senses.Methods: In this study, we compared adults with ASD (n = 29), SZ (n = 24), and typical developmental histories (TD, n = 37) on two tasks requiring visual-tactile spatial multisensory processing. In the first task (crossmodal congruency), participants judged the location of a tactile stimulus in the presence or absence of simultaneous visual input that was either spatially congruent or incongruent, with poorer performance for incongruence an index of spatial multisensory interaction. In the second task, participants reacted to touch in the presence or absence of dynamic visual stimuli that appeared to approach or recede from the body. Within a certain radius around the body, defined as peripersonal space (PPS), an approaching visual or auditory stimulus reliably speeds reaction times (RT) to touch; outside of this radius, in extrapersonal space (EPS), there is no multisensory effect. PPS can be defined both by its size (radius) and slope (sharpness of the PPS-EPS boundary). Clinical measures were administered to explore relations with visual-tactile processing.Results: Neither clinical group differed from controls on the crossmodal congruency task. The ASD group had significantly smaller and more sharply-defined PPSs compared to the other two groups. Small PPS size was related to social symptom severity across groups, but was largely driven by the TD group, without significant effects in either clinical group.Conclusions: These results suggest that: (1) spatially static visual-tactile facilitation is intact in adults with ASD and SZ, (2) spatially dynamic visual-tactile facilitation impacting perception of the body boundary is affected in ASD but not SZ, and (3) body boundary perception is related to social-emotional function, but not in a way that maps on to clinical status.

Robotic hand illusion with tactile feedback: Unravelling the relative contribution of visuotactile and visuomotor input to the representation of body parts in space

The rubber hand illusion describes a phenomenon in which participants experience a rubber hand as being part of their body by the synchronous application of visuotactile stimulation to the real and the artificial limb. In the recently introduced robotic hand illusion (RobHI), a robotic hand is incorporated into one’s body representation due to the integration of synchronous visuomotor information. However, there are no setups so far that combine visuotactile and visuomotor feedback, which is expected to unravel mechanisms that cannot be detected in experimental designs applying this information in isolation. We developed a robotic hand, controlled by a sensor glove and equipped with pressure sensors, and varied systematically and separately the synchrony for motor feedback (MF) and tactile feedback (TF). In Experiment 1, we implemented a ball-grasping task and assessed the perceived proprioceptive drift of one’s own hand as a behavioral measure of the spatial calibration of body coordinates as well as explicit embodiment experiences by a questionnaire. Results revealed significant main effects of both MF and TF for proprioceptive drift data, but we only observed main effects for MF on perceived embodiment. Furthermore, for the proprioceptive drift we found that synchronous feedback in one factor compensates for asynchronous feedback in the other. In Experiment 2, including a new sample of naïve participants, we further explored this finding by adding unimodal conditions, in which we manipulated the presence or absence of MF and/or TF. These findings replicated the results from Experiment 1 and we further found evidence for a supper-additive multisensory effect on spatial body representation caused by the presence of both factors. Results on conscious body perception were less consistent across both experiments. The findings indicate that sensory and motor input equally contribute to the representation of spatial body coordinates which for their part are subject to multisensory enhancing effects. The results outline the potential of human-in-the-loop approaches and might have important implications for clinical applications such as for the future design of robotic prostheses.

The eardrums move when the eyes move: A multisensory effect on the mechanics of hearing

Interactions between sensory pathways such as the visual and auditory systems are known to occur in the brain, but where they first occur is uncertain. Here, we show a novel multimodal interaction evident at the eardrum. Ear canal microphone measurements in humans (n = 19 ears in 16 subjects) and monkeys (n = 5 ears in 3 subjects) performing a saccadic eye movement task to visual targets indicated that the eardrum moves in conjunction with the eye movement. The eardrum motion was oscillatory and began as early as 10 ms before saccade onset in humans or with saccade onset in monkeys. These eardrum movements, which we dub Eye Movement Related Eardrum Oscillations (EMREOs), occurred in the absence of a sound stimulus. The EMREOs’ amplitude and phase depended on the direction and horizontal amplitude of the saccade. They lasted throughout the saccade and well into subsequent periods of steady fixation. We discuss the possibility that the mechanisms underlying EMREOs create eye movement-related binaural cues that may aid the brain in evaluating the relationship between visual and auditory stimulus locations as the eyes move.

The eardrums move when the eyes move: A multisensory effect on the mechanics of hearing

Interactions between sensory pathways such as the visual and auditory systems are known to occur in the brain, but where they first occur is uncertain. Here, we show a multimodal interaction evident at the eardrum. Ear canal microphone measurements in humans (n = 19 ears in 16 subjects) and monkeys (n = 5 ears in three subjects) performing a saccadic eye movement task to visual targets indicated that the eardrum moves in conjunction with the eye movement. The eardrum motion was oscillatory and began as early as 10 ms before saccade onset in humans or with saccade onset in monkeys. These eardrum movements, which we dub eye movement-related eardrum oscillations (EMREOs), occurred in the absence of a sound stimulus. The amplitude and phase of the EMREOs depended on the direction and horizontal amplitude of the saccade. They lasted throughout the saccade and well into subsequent periods of steady fixation. We discuss the possibility that the mechanisms underlying EMREOs create eye movement-related binaural cues that may aid the brain in evaluating the relationship between visual and auditory stimulus locations as the eyes move.

Upper limb cortical maps in amputees with targeted muscle and sensory reinnervation.

Neuroprosthetics research in amputee patients aims at developing new prostheses that move and feel like real limbs. Targeted muscle and sensory reinnervation (TMSR) is such an approach and consists of rerouting motor and sensory nerves from the residual limb towards intact muscles and skin regions. Movement of the myoelectric prosthesis is enabled via decoded electromyography activity from reinnervated muscles and touch sensation on the missing limb is enabled by stimulation of the reinnervated skin areas. Here we ask whether and how motor control and redirected somatosensory stimulation provided via TMSR affected the maps of the upper limb in primary motor (M1) and primary somatosensory (S1) cortex, as well as their functional connections. To this aim, we tested three TMSR patients and investigated the extent, strength, and topographical organization of the missing limb and several control body regions in M1 and S1 at ultra high-field (7 T) functional magnetic resonance imaging. Additionally, we analysed the functional connectivity between M1 and S1 and of both these regions with fronto-parietal regions, known to be important for multisensory upper limb processing. These data were compared with those of control amputee patients (n = 6) and healthy controls (n = 12). We found that M1 maps of the amputated limb in TMSR patients were similar in terms of extent, strength, and topography to healthy controls and different from non-TMSR patients. S1 maps of TMSR patients were also more similar to normal conditions in terms of topographical organization and extent, as compared to non-targeted muscle and sensory reinnervation patients, but weaker in activation strength compared to healthy controls. Functional connectivity in TMSR patients between upper limb maps in M1 and S1 was comparable with healthy controls, while being reduced in non-TMSR patients. However, connectivity was reduced between S1 and fronto-parietal regions, in both the TMSR and non-TMSR patients with respect to healthy controls. This was associated with the absence of a well-established multisensory effect (visual enhancement of touch) in TMSR patients. Collectively, these results show how M1 and S1 process signals related to movement and touch are enabled by targeted muscle and sensory reinnervation. Moreover, they suggest that TMSR may counteract maladaptive cortical plasticity typically found after limb loss, in M1, partially in S1, and in their mutual connectivity. The lack of multisensory interaction in the present data suggests that further engineering advances are necessary (e.g. the integration of somatosensory feedback into current prostheses) to enable prostheses that move and feel as real limbs.

Auditory-visual integration modulates location-specific repetition suppression of auditory responses.

Space is a dimension shared by different modalities, but at what stage spatial encoding is affected by multisensory processes is unclear. Early studies observed attenuation of N1/P2 auditory evoked responses following repetition of sounds from the same location. Here, we asked whether this effect is modulated by audiovisual interactions. In two experiments, using a repetition-suppression paradigm, we presented pairs of tones in free field, where the test stimulus was a tone presented at a fixed lateral location. Experiment 1 established a neural index of auditory spatial sensitivity, by comparing the degree of attenuation of the response to test stimuli when they were preceded by an adapter sound at the same location versus 30° or 60° away. We found that the degree of attenuation at the P2 latency was inversely related to the spatial distance between the test stimulus and the adapter stimulus. In Experiment 2, the adapter stimulus was a tone presented from the same location or a more medial location than the test stimulus. The adapter stimulus was accompanied by a simultaneous flash displayed orthogonally from one of the two locations. Sound-flash incongruence reduced accuracy in a same-different location discrimination task (i.e., the ventriloquism effect) and reduced the location-specific repetition-suppression at the P2 latency. Importantly, this multisensory effect included topographic modulations, indicative of changes in the relative contribution of underlying sources across conditions. Our findings suggest that the auditory response at the P2 latency is affected by spatially selective brain activity, which is affected crossmodally by visual information.

‘The Pictures are Better on Radio’: A Visual Analysis of American Radio Drama from the 1920s to the 1950s

This essay takes a sensory approach to the study of radio listening by looking at American radio dramas of the 1920s through the 1950s. Analysis of radio writing handbooks, fan magazines, publicity and radio content from the period will show that radio producers, writers and actors conceptualised the art of radio listening as acutely visual, despite its auditory format. The medium invited the listener to participate actively in the visualisation process, to formulate mental pictures of characters, settings and scenes through the personalised act of listening. And yet, radio practitioners endeavoured to guide their listeners towards similar visual conceptions of the fictional worlds of radio drama by drawing upon familiar images and stereotypes already in broad circulation. To do this, they created a multi-sensory effect by layering sound, combining speech, silence, music and sound effects to kindle the imagination of listeners. They utilised identifiable dialects, music and language that correlated to stereotypical depictions of race, gender, class, ethnicity, nationality and place in order to create a shared portrait of characters or settings. This essay will also consider the impact of the concurrent transmission of dramatic radio content on multiple media platforms. The sensory experience of listening was no longer strictly auditory when audiences received additional visual cues through other media. Because many radio shows also had counterparts in cinema, novels, comic strips and later television shows, the process of visualising radio content for listeners could not be detached from the experiences of engaging its stories through other sensory formats. Listeners could also gain visual familiarity with actors and settings by perusing fan magazines or seeing promotional advertising. The conscious reliance on images broadly shared in the media culture as well as the multi-media consumption of radio content allowed for a listening experience that was as much unifying as individualised.

Bill Fontana’s Distant Trains: A documentation of an acoustic relocation

Witnessing a sound installation in person offers an opportunity to experience the qualities and elements of a work first hand and in full, multisensory effect. A thorough documentation of an exhibition and the work that goes into it is at the essence of preserving important information for future generations. Though information can be gathered from archives, some works of sound art are only marginally presented in the literature, making it difficult to fully grasp aspects of an artist’s technical, organisational and, most particularly, creative ways of working. Instead, already existing information is often reproduced. Previous documentation regarding Bill Fontana’s Sound Sculpture Distant Trains, exhibited in Berlin in 1984, offers an example of the possible loss of key details. This article aims to present new research findings that will examine and illuminate the full scope of this artistic project.

Hemispheric asymmetry: Looking for a novel signature of the modulation of spatial attention in multisensory processing

The extent to which attention modulates multisensory processing in a top-down fashion is still a subject of debate among researchers. Typically, cognitive psychologists interested in this question have manipulated the participants’ attention in terms of single/dual tasking or focal/divided attention between sensory modalities. We suggest an alternative approach, one that builds on the extensive older literature highlighting hemispheric asymmetries in the distribution of spatial attention. Specifically, spatial attention in vision, audition, and touch is typically biased preferentially toward the right hemispace, especially under conditions of high perceptual load. We review the evidence demonstrating such an attentional bias toward the right in extinction patients and healthy adults, along with the evidence of such rightward-biased attention in multisensory experimental settings. We then evaluate those studies that have demonstrated either a more pronounced multisensory effect in right than in left hemispace, or else similar effects in the two hemispaces. The results suggest that the influence of rightward-biased attention is more likely to be observed when the crossmodal signals interact at later stages of information processing and under conditions of higher perceptual load—that is, conditions under which attention is perhaps a compulsory enhancer of information processing. We therefore suggest that the spatial asymmetry in attention may provide a useful signature of top-down attentional modulation in multisensory processing.

Design and Assessment of a Virtual Underwater Multisensory Effects Reproducing Simulation System

With current developments in virtual reality technology and advances in the marine industry, various virtual underwater simulation systems that help users to indirectly experience the marine environment via a variety of virtual contents are being developed. In general, underwater simulation systems that use virtual reality developed using three-dimensional graphics technology. Therefore, they are limited in terms of their ability to imbue users with a sense of immersion and presence. In addition, the expansion of the sensory range, with which users can actually experience various underwater environmental phenomena while they are in underwater simulations, is still insufficient. This paper proposes an immersive multisensory effect reproduction system that provides an improved sense of underwater reality for users. The results of comparative user evaluation tests conducted regarding the sense of reality imbued with our proposed system indicate that its sensory presence effect is superior to that of conventional systems.

The effect of task-irrelevant stimulus ‘graspability’ on reaching actions

Reaching and grasping requires integration of visual, proprioceptive and somatosensory inputs. Previous research has shown that manipulating the ‘graspabilty’ of a visual stimulus influences reaction times to that stimulus (e.g., Tucker and Ellis, 1998). Here we explored whether this same effect can be extended to the planning and online control of arm movements. Participants made a mimed reaching movement with their left or right hand depending on the colour of images of affordance (door handles) and control stimuli (a row of dots of similar size and orientation as the door handle). Stimulus onset was manipulated by changing when the grey stimulus changed colour. Stimuli either pointed towards (compatible) or pointed away from (incompatible) the responding hand. Spatially compatible affordance stimuli facilitated reach onset compared to other stimuli and compatibility combinations, replicating previous reaction time studies. This can be attributed to a priming of the motor system by spatially compatible affording items. Results also indicated a larger outwards deviation of reach trajectory for spatially incompatible control stimuli compared to spatially compatible control stimuli, which waned with stimulus onset delay. This reveals an immediate inhibitory effect on reach trajectory, such that outwards movement is over-compensated to negate this incompatible orientation. Overall, we observed that the effect of visual spatial compatibility on reach kinematics differs with the action relevance of the stimulus. We are currently exploring how this multisensory visuomotor effect changes with age.

Multisensory enhancement of attentional capture in visual search

Multisensory integration increases the salience of sensory events and, therefore, possibly also their ability to capture attention in visual search. This was investigated in two experiments where spatially uninformative color change cues preceded visual search arrays with color-defined targets. Tones were presented synchronously with these cues on half of all trials. Spatial-cuing effects indicative of cue-triggered capture of attention were larger on tone-present than on tone-absent trials, demonstrating multisensory enhancements of attentional capture. Larger capture effects for audiovisual events were found when cues were color singletons, and also when they appeared among heterogeneous color distractors. Tone-induced increases of attentional capture were independent of color-specific top-down task sets, suggesting that this multisensory effect is a stimulus-driven bottom-up phenomenon.

Seeing the hand boosts feeling on the cheek

Tactile acuity improves when subjects look at the stimulated body part, even when vision does not provide any information relevant to touch. This “visual enhancement of touch” (VET), might involve modulation of primary somatosensory cortex (SI) processing by multimodal information related to the body. SI shows a characteristic somatotopic organization, with the face and hand represented laterally and adjacent to each other, and the foot represented more medially. The aim of this study was to investigate whether VET is limited to the viewed body part, spreads to all body parts, or generalizes only to body parts represented in SI closely to the viewed body part. Tactile acuity was assessed in healthy subjects on the hand, face and foot, while subjects viewed either their stimulated hand, their foot or were blindfolded: viewing the hand, compared to blindfold condition, enhanced tactile sensitivity on the hand and also on the face, but not on the foot. Conversely, viewing the foot, compared to blindfold condition, improved touch only on the foot. Two control experiments assured this effect was due to viewing the body and not to directing visuo-spatial attention toward the location of tactile stimulation. The present results show that VET acts accordingly to a somatotopic gradient based on SI organization, suggesting that this multisensory effect may occur within SI. This finding might have a possible application to facilitate the recovery of tactile deficits in patients with a lesion of somatosensory cortices.